1. Technical Field
The present invention relates to a printing apparatus for correcting input density to perform printing, a printing method, an image processing apparatus, an image processing method, a printing program, an image processing program, and a storage medium.
2. Related Art
An ink-jet printer that discharges ink droplets on a sheet, serving as a medium, to form dots has been known as a printing apparatus for printing images. The ink-jet printer repeatedly performs a dot forming operation for discharging ink droplets from a plurality of nozzles that move in a predetermined moving direction to form dots on a sheet and a sheet transfer operation for transferring the sheet in a direction intersecting the moving direction of the sheet (hereinafter, referred to as a transfer direction) by using a transfer unit. In this way, a plurality of raster lines each composed of a plurality of dots formed along the moving direction are formed in the transfer direction, thereby printing an image.
However, in this type of printer, irregularity in the density of an image composed of a plurality of raster lines in parallel to the moving direction occurs. That is, dark raster lines and light raster lines may macroscopically appear.
The irregularity in the density is generally caused by the processing accuracy of nozzles, and the following method is used to control the irregularity in the density. First, a printer that controls the density irregularity prints a correcting pattern (test pattern), and a density measuring apparatus measures the density of the pattern to calculate the correction value of each nozzle so as to obtain a target density. Then, the correction value of each nozzle is stored in a storage unit of the printer. When a user prints an image, the printer uses the correction value corresponding to each nozzle to correct the discharge amount of ink, thereby controlling the density irregularity. For example, JP-A-5-92559 discloses a printing apparatus for correcting the input density of each raster line to perform printing.
In the printing apparatus, read image data that is read by a document reading device and corresponds to each recording element of a recording head is obtained by weighting each image data to image data of another recording element in a smoothing circuit. Therefore, a CPU calculates the correction value of each recording element on the basis of the image data, the calculated values are stored in an irregularity correcting data backup RAM, and a correcting circuit uses the correction values to correct the image data. A binarizing circuit is a recording device for generating recording head driving data by weighting correcting image data of other recording elements on the basis of the corrected image data of each recording element. In this way, when a recording head including a plurality of recording elements is used to print an image, it is possible to rapidly and exactly correct the irregularity in the density caused by the recording elements.
However, according to the printing apparatus disclosed in JP-A-5-92559, it is possible to reduce the irregularity in the density, but it is difficult to reduce the banding of grayscale levels which are visually noticeable to the human eye. That is, it does not matter if the correction accuracy is low in a highlighted region that is not visually noticeable to the human eye. Therefore, it is possible to reduce the number of grayscale levels of a test pattern, but it is difficult to improve the accuracy of density correction in a grayscale region (shadow region) that is visually noticeable to the human eye.